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Blewbyu
06-22-2000, 07:34 PM
Somewhere in the Donzi pages I read about the dreaded reversion- and as I recall it is caused by cam configuration.I don't understand cams-and I don't really care to as long as they make the motor hump out torque. Mine does- it's a Chevy crate motor they call the H.O. Bowtie smallblock- and the package creates 300 lb ft @ 1700 rpm/369 lb ft @ 3600, and 345 hp @ 5600.So far this engine has lived thru almost 350 hrs in the X-18 with no obvious signs of distress.The cam is described as "High Perfomance hydraulic roller profile with 235 degrees duration (@.050 tappet lift)and .480 maximum valve lift". Big ?- is this one of those sucky type reversion cams and I've just been lucky so far....or is it one that blows all the time?????? If I lost one of the exterior rubber flappers- dare I back into a wave without fear of reversion??? Bottom line as I read it...Reversion Sucks!
Regards- Uncle Jeffrey

PaulO
06-23-2000, 08:37 AM
Jeff,
That is not a particularly "large" cam although it is "larger" than stock marine cams. What really matters though, is the overlap. This is the time, measured in degrees, that the exhaust valve and the intake valve are open simultaneously. Theoretically, this can allow a momentary suction into the exhaust manifold. Whether or not you would experience reversion would depend on how much vacuum and how far downstream in the exhaust the cooling water is introduced. If you have 350 hours and no ill effects, you are probably OK. If you had reversion you would have excessive steam at low speed, really clean white spark plugs, possibly hydro-lock, and in extreme cases, water in the oil.

As far as the flapper goes, that is non-related. You can easily force seawater up the exhaust and into the manifold where it will find its way into the cylinders by backing into a wave or slowing too abruptly.
Hope this helps,
PaulO

Jamesbon
06-23-2000, 08:51 AM
Uncle Jeffrey,
I recalled an article discussing Intake reversion, but am not sure how well it relates to exhaust reversion at http://www.iskycams.com/techtips.html#2002 .

Another factor to consider is your exhaust manifold/riser setup. I run a stout 305 degree duration cam in conjunction with long tube risers. My "Long tube" risers inject water into the exhaust gases almost 2 feet from the exhaust manifold, thus eliminating the chance of water being sucked into the exhaust port.

Take a look at the specs. for "marine cams" on any reputable cam manufacturers website, and compare those specs to yours. My gut feeling re: your cam, is that you're okay.

Blewbyu
06-23-2000, 10:06 PM
Thanks gentlemen-I never did understand "overlap".The way you explained it, it doesn't sound complex- even if cam design is complex.Seems to make sense. Backing or sudden deceleration has never been an issue with my other boats, in that none had thru transom above water exhaust.I'll not be backing hard and will be more careful of deceleration, thank you very much!
While on the subject- what constitutes a "large" cam? Lift, duration, or both? AND- does a "large" cam create mainly torque or horsepower? Oooops,I believe I said I didn't WANT to know about cams...guess I need to know just enuf to partially understand- but not enuf to be conversational.Does the "duration" figure address only exh. valve opening?
Best Regards- Jeff

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AVickers
06-24-2000, 11:36 AM
Valve overlap (both valves being open at the same time) allows a "scavenging effect" in that the outgoing pulse of exhaust gases draw intake gases into the cylinder, thus making for a more dense fill during the compression stage. This scavenging makes for a more efficient engine at certain mid-to-higher RPMS as the velocity of the gases, timing of the movement of gas columns (in the intake runner, the cylinder and the exhaust runner), and runner lengths and diameters become factors in the movement of gasses through the engine – air/fuel mix coming in; burning gasses during combustion w/ associated flame-front; and exhaust gasses going out. Anyway, with all this, you essentially “tune” these things to produce the maximum power at a certain RPM range and everything moves through quite well at the given range. The cam is the “switch” that times all these events.

At the given RPM, overlap is GOOD. Unfortunately, at other RPMs (like idle), this can be BAD. Valve overlap allows scavenging, but it also allows the exhaust valve to be open as the piston begins heading down and thus creating a vacuum in the cylinder. At very low RPM, this valve open/piston heading down situation can overcome the scavenging effect of the gasses flowing through the engine and it literally will suck a bit of exhaust gas back into the cylinder at the very end of the exhaust stroke. No big deal on a car engine ‘cause it’s just sucking gasses – besides, that’s what makes a mildly radically cammed engine sound so good… It’s also why really radically cammed engines have to idle so high...given enough exhaust reversion, they won’t idle at all… At higher RPMs, even off idle, the scavenge effect overcomes the vacuum of the piston beginning to head down in the cylinder.

ON A BOAT, where water is being introduced into the exhaust stream (sometimes within inches of the exhaust port itself), reversion can become deadly. At idle, it could suck water back into a hot engine, thus creating immediate release of steam with a commensurate radical drop in temperature and changes in the geometry and metallurgy of mechanical componentry – like warped valves and piston lands or annealed seats. Or, in the worst case, liquid could be introduced into the cylinder of a running engine (and we all know that water doesn’t compress) and you could bend or break something...or, as they say in engineering school, radically displace the geomety of a component.

That’s why drag boats – with radical cams – introduce water into the headers at the collector, not at the runner. This way, they can idle (as much as a radically cammed engine can) and not worry too much about reversion... Bottom line is that in marine application, you need to be assured that the cam/intake/exhaust setup will produce some scavenging even at idle if an exhaust valve is open while the piston is headed down… OR, if reversion will occur, you need to be sure that water is being introduced way downstream of the exhaust port in your particular collector/riser configuration.

Now, if we could just get a cam that changes its timing characteristics at different RPMs... Oops, somebody's already done that. Nissan's ZX engine has variable valve timing, so does Honda and Toyota and BMW and Porsche and almost all the other guys.

What we boater guys need is a decently sized V8 with this feature. Anybody got a 4.7L Toyota engine they want to donate to my variable-valve-timing-in-a-Donzi experiment????